This invention relates to microstrip gas chambers used to detect the presence of radiation such as x-rays and charged atomic particles.
A microstrip gas chamber (MSGC) was first proposed by A. Oed. The principle of MSGC is similar to that of multiwire proportional chambers. Referring to FIG. 1, the MSGC includes a glass substrate, a sequence of alternating cathode strips and anode strips printed by photolithographic techniques on a surface of the substrate. The MSGC is enclosed in a gas volume and voltages are applied to the electrodes (anode strips and cathode strips) to produce an electric field across the gas volume. A charged particle moving through the gas volume causes ionization along its path, creating ion pairs. The electric field across the gas volume causes the electrons to drift toward the anode strips and positive ions toward the cathode strips, so developing an electrical signal. The electric field near the anode strips is sufficiently high for the electrons drifting towards it to accelerate to a point where they themselves are capable of causing further ionization, causing an avalanche of electrons, which amplify the original signal. The increase in the number of electrons detected at the anode strips for each secondary electron entering the avalanche region is known as the gas gain (the multiplication of the signal).
For further description related to the principle of MSGC, see U.S. Pat. No. 5,675,470 issued to Wen G. Gong, U.S. Pat. No. 5,614,722 issued to Keith Solberg, et al., and U.S. Pat. No. 5,500,534 issued to Douglas S. Robinson, et al. which are incorporated herein by reference.
This type of gas chamber has many attractive properties such as excellent mechanical stability, small anode-cathode distance, fast ion collection time, etc.
However, a charge-up problem on the insulating substrate has been observed in MSGC as shown in FIG. 2(A) and it has spoiled better intrinsic properties of MSGC. The accumulation of positive ions on the surface of the substrate between anode and cathode is known to be responsible for gain instability in the MSGC. The positive ions produced due to avalanche amplification occurring in the vicinity of the anode electrodes stick to the surface of the substrate and cause a space-charge effect, modifying the electric field, and resulting in gain instability and a decreased gain.
The surface charge in the MSGC can be removed by providing the slight conductivity to the substrate. This slight conductivity should he chosen to be quite high in order to minimize the electric field distortion in the MSGC and keep a high gas amplification of the MSGC. However, this conductivity might still affect the increasing probabilities of the streamers which are thought to be originated from the electron emission at the cathode surface. The conductivity enhances the lateral field near the substrate surface between the anode and the cathode that may cause streamer discharge, result in breakage of electrodes. Therefore, the possibilities of streamers limit the maximum amplification factors attained without any possible damage to the electrodes.
Many works have been already attempted to optimize the surface conductance and the electrode shapes or even remove the substrate. One effective method is to use a so-called small gap distance between the anode and the cathode as shown in FIG. 2 (B). Recently, many researchers are interested in the small gap MSGC because this type of detector relieves the surface charge-up problem and provide a better stability in operation.
This approach, however, suffers from the poor gas gain of the MSGC. This structure has a serious drawback in its gas gain in that the gain is usually restricted within several hundreds to one thousand. If a higher voltage is applied to a gap between each anode strip and the corresponding cathode strip, discharge is likely to occur between the anode strip and the cathode strip, resulting in breakage of the electrodes. To improve the obtainable gas gain, some groups used an additional insulator to isolate neighboring electrodes to prevent discharge between the anode and cathode.
The present invention seeks to eliminate those drawbacks of the conventional MSGC.
An object of the present invention is to eliminate the charge build-up on the surface of a substrate between anode and cathode strips.
Another object of the present invention is to prevent the occurrence of discharge between the anode strip and the cathode strips.
Still another object of the present invention is to provide a MSGC having the high gain and the stable operation.
In accordance with the present invention, there is provided a microstrip gas chamber comprises a gas volume, an electrically insulating substrate having a surface exposed to the gas volume, a set of alternating cathode strips and anode strips on the surface of substrate, a high voltage source for establishing a potential difference between the anode and cathode strips to thereby produce an electric field sufficient for avalanche multiplication, in the gas medium in a region near the anode strips, and at least one grid electrode provided on the surface at each gap between the cathode strip and anode strip.
In accordance with the present invention, an exposed area between the anode and cathode strips is decreased or minimized by providing the grid at the gap between the anode strip and cathode strips so that the accumulation of positive ions on the exposed surface is eliminated.
Preferably, the grid is a strip extending along the lengths of anode and cathode strips. Preferably, a plurality of grid strips are provided at the gap between the anode strip and cathode strip. The grid may be comprised of a plurality of grid elements (a short strip, for example) which are spacedly arranged to each other along the lengths of anode strip and cathode strip.
In accordance with the present invention, the grid eliminates or shields the lateral electric field near the surface of the substrate between the neighboring anode and cathode strips. Though the grid may somehow work without positively applying any potential to the grid, most preferably, a predetermined voltage is applied to the grid. The predetermined voltage may be different than those applied to the anode and cathode strips. More specifically, the predetermined voltage is higher than that applied to the cathode strips, and is lower than that applied to the anode strips. In case of providing multiple strip grids at the gap between the anode strip and cathode strip, individual voltages may be applied to the grids and respective high voltage sources may be provided.
To shield the lateral electric field between the neighboring anode and cathode strips near the surface effectively, a height of grid electrode may be higher than those of anode and cathode strips though the fabrication may be more complicated. In case of providing one grid between the gap for example, the grid electrode may be provided near the anode strip and a relatively wider exposed surface area is presented between the grid electrode and the cathode strip since the gas gain is mostly determined by the electric field near the anode strip and surface charges between the anode strip and the grid nearest to the anode strip.